The invention relates to a display device comprising an image display panel having a first substrate which is provided with light-reflecting electrodes at the area of pixels, an illumination system comprising an optical waveguide of an optically transparent material having an exit face facing the image display panel and a plurality of end faces, at least one of said end faces being an entrance face for light, while light can be coupled into said end face of the optical waveguide.
The image display panel may comprise an electro-optical medium (between two substrates) such as liquid crystalline material or an electrochromic material. It may also be based on electrostatic forces (deformable mirrors).
The invention also relates to an illumination unit (or front light) for such a display device and to methods of manufacturing such illumination units.
Such reflective display devices are used in, for example, portable apparatus such as laptop computers, mobile telephones, personal organizers, etc. With a view to saving energy, it is desirable that the light source can be switched off in the case of sufficient ambient light.
A display device of the type mentioned above is described in WO 99/22268. In the optical waveguide described in this document, an unpolarized beam from the light source as split up into two mutually perpendicularly polarized beam components. Polarization separation is obtained by causing the unpolarized beam to be incident on an interface between an area of isotropic material having a refractive index np and an area of anisotropic material having refractive indices no and ne, in which one of the two indices no or ne is equal or substantially equal to np. When an unpolarized beam is incident on such an interface, the beam component which does not experience any refractive index difference at the transition between isotropic and anisotropic material is passed in an undeflected form, whereas the other beam component is deflected or reflected. One of the two beam components is subsequently passed by a polarizer to a reflective liquid crystal panel. The optical waveguide shown exhibits much less image distortion than a known optical waveguide with a groove structure (microprisms) on the viewing side of the optical waveguide. The image distortion is produced because the groove structure has different slopes, which results in multiple image formation. Generally, this multiple image formation is prevented by providing an optical compensator having a complementary groove structure.
However, in the display device described in WO 99/22268, stray light is generated in the viewing direction on the interface between the areas with isotropic and anisotropic material.
Moreover, the light which is deflected in the direction of the image display panel sometimes undergoes partial reflections in the image display panel and in the optical waveguide before the light reaches the reflecting pixels.
These drawbacks apply to the same or an even greater extent to optical waveguides which are based on a groove structure.
It is, inter alia, an object of the present invention to provide a solution to the above-mentioned problem.
To this end, a display device according to the invention is characterized in that the optical waveguide is present between the image display panel and a circular polarizer, and the optical waveguide comprises polarizing means for substantially circularly polarizing the entering light. In this application, the word circular is also understood to be xe2x80x9cellipticalxe2x80x9d. In certain circumstances (when less contrast is sufficient) it is also possible to work with elliptically polarized light.
The polarizer may be integrated in the display device.
The polarizing means have a similar function as in the known device, namely polarizing light rays from the light source, in which light of one kind of polarization (for example, levorotatory polarization) is deflected in the direction of the image display panel. In the relevant case, light exiting on the viewing side (dextrorotatory polarized light in the same example) is not passed by the polarizer.
Due to polarization, an unpolarized beam from the light source is split up into two mutually oppositely polarized beam components (levorotatory and dextrorotatory). Such a polarization separation is obtained, for example, by causing the unpolarized beam to be incident on an interface between an area of isotropic material and an area of chiral nematic material, for example, a chiral nematic liquid crystal material provided in, for example, a groove structure, or a (patterned) chiral nematic network. When an unpolarized beam is incident on such an interface, a beam component of one handedness is passed undeflected on the transition between isotropic and chiral nematic material, while the beam component having the other, opposite handedness is deflected or reflected.
A suitable embodiment is characterized in that the pitch of the chiral nematic liquid crystal material or the chiral nematic polymer network within a groove varies. A larger bandwidth of the reflected light can thereby be obtained.
A first method of manufacturing such an illumination unit (or front light) provided with polarizing means for circularly (or elliptically) polarizing the entering light is characterized in that a surface of a transparent body is provided with grooves, and the transparent body within the grooves is provided with a chiral nematic liquid crystal material or the chiral nematic polymer network.
A second method is characterized in that a surface of a transparent body is provided with a layer of a chiral nematic material which is locally converted into isotropic material.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.